Boeing's 30,000-pound Massive Ordnance Penetrator (MOP), an ultra-large bunker buster for use on underground targets, with Iran routinely mentioned as its most likely intended destination, is a key element in the implicit U.S. threat to use force as a last resort against Iran's nuclear ambitions.

Would that weapon, delivered in a gouging combination with other precision-guided munitions, pulverize enough rock to reach down and destroy the uranium enrichment chamber sunk deep in a mountain at Fordow, Iran's best sheltered nuclear site?

Critics of Iran's nuclear program tend to agree that military action against Iran's nuclear work would be their last and worst option. Not only would this risk civilian casualties, but Iran would seek to retaliate against Western targets in the region, raising the risk of a regional war and risking global economic turmoil.

Once it had recovered it would probably decide unequivocally to pursue a nuclear bomb.

GBU-57A/B Massive Ordnance Penetrator

John Cochrane, a defense specialist at the London-based Exclusive Analyst risk consultancy, said he believed the bunker-busting MOP might make a difference. But he suggested Fordow was at the very limit of the bomb's capacities, which he said could reach down to a maximum of 60 meters.

"Repeated strikes by Tomahawk cruise missiles and MOP might be effective in penetrating the site, if it is not as deep as 80m but, even then, we question whether an attack would have the same level of assurance in terms of damage as strikes on other 'softer' sites," he told Reuters.

Cyber attack or physical assault by Special Forces may be the only attack options.

Doing it the hard way

In a November 2011 article in Israel's Tablet magazine, Columbia University's Long concluded that Israel had the ability to attack the Fordow site using 75 bunker busters, each delivering a smaller explosive charge of about 1,000 pounds. However, he said it would require an unprecedented level of precision.

Long's scenario sees Israeli jets having "to do things the hard way," delivering 75 bunker busters on a single point to burrow through the rock.

There were two principal challenges, he said.

First, the weapons themselves, dropped from miles away and thousands of feet in the air, had to arrive at very close to the same angle to create a pathway each subsequent weapon could follow, he wrote. "Otherwise much of the penetrating power of the bombs will be wasted."

The second unknown was the "spoil problem," where the sides of the pathway, destroyed by previous explosions, clog the pathway for subsequent bombs.